Fluorocarbon resins, such as polyvinylidene fluoride (PVDF), are useful in formulating coatings with excellent weathering resistance. Because fluorocarbons have poor rheology and pigment wetting characteristics, it is common to add a modifying polymer to fluorocarbon coating compositions. A typical binder polymer is an acrylic polymer, and both thermosetting and thermoplastic acrylics resins have been utilized for this purpose.
Coatings containing fluorocarbons and thermosetting resins, wherein the acrylic polymers have functionality that renders them cross-linkable, and in which a cross-linker is included in the resin composition, are sold by several coating producers. U.S. Pat. No. 3,779,976 describes combinations of polyvinyl fluoride and a thermoplastic copolymer of methyl methacrylate and 3-(2-methacryloxyethyl)-2,2-spirocyclohexyl oxazolidine. Thermosetting binder polymers are preferred for many applications because of the improved mechanical and chemical resistance they provide.
As noted in U.S. Pat. No. 5,185,403, a high fluorocarbon resin content is required to achieve optimal weathering and chemical resistance. Many coating applications call for coating compositions in which the fluorocarbon resin comprises 70 wt. % or more of the resin and the binder polymer the remainder. Coating compositions containing fluorocarbon resins, particularly PVDF, and thermosetting acrylic resins tend, however, to have relatively high viscosities at the solids levels used. For some coating applications, particularly coil coating, it is desirable to have high fluorocarbon resin content, a high total solids content and a lower viscosity than is currently available.
In a coil coating operation, a coil of sheet metal is uncoiled as it is pulled through a series of rollers, one or more of which is a paint applicator roller, at up to 1000 feet per minute. The paint is picked up by a roller rotating in a paint pan and transferred to an applicator roller, thence to the moving sheet metal. It is then passed through a curing oven and coiled again for the market. The cost of coating could be lessened greatly if the solids content of the paint were high enough that a sufficiently thick layer could be applied in one pass through the coating process. But a high solids content usually means that the paint's viscosity would be too great for efficient transfer from roller to roller.